Number P0638: Throttle Actuator Control Range/Performance Bank 1
The electronic throttle motor positions the throttle plate; P0638 means the actuator does not follow its command on bank 1 (or the only throttle): target and actual angle from the two position sensors differ, or the motor needs too much duty cycle to hold the position. Subaru (also sold as Saab 9-2X
Can you drive with P0638?
Severity: Fix promptly — drivability fault. Short trips at moderate speed only. If the car has entered limp mode (won't rev, stuck in one gear), have it towed rather than forcing it.
If you ignore it: Limp-mode faults tend to strand you, and several of them (misfires, throttle faults) damage other parts while you wait.
This is the #117 most common code in our top-150 library, in the idle & throttle control family (Powertrain system). Severity is Number Nest's editorial triage — if the light is flashing or the car feels unsafe, stop driving it whatever any page says.
What P0638 means
The electronic throttle motor positions the throttle plate; P0638 means the actuator does not follow its command on bank 1 (or the only throttle): target and actual angle from the two position sensors differ, or the motor needs too much duty cycle to hold the position. Subaru (also sold as Saab 9-2X) sets it at more than 3.5° deviation for 250 ms or over 95 % duty cycle for 2 s, lights the lamp at once and lets the spring set the plate to about 6° – raised idle, almost no power. GM's Saab 9-4X V6 uses more than 80 % duty cycle for over 1 s, lamp in the first trip. Mazda's CX-9 set it after frosty nights when condensation froze the throttle and fixed it with software that moves the plate after switch-off. Ford reports the same function as P2111/P2112.
Systems involved: throttle body, throttle actuator motor, engine control module (ECM/PCM).
Causes, ranked by likelihood
| # | Cause | Likelihood |
|---|---|---|
| 1 | Throttle dirty, sooted or stiff | most likely |
| 2 | Icing from condensation in frost | possible |
| 3 | Throttle motor or gears worn | possible |
| 4 | Resistance in the motor control wires | possible |
| 5 | Module or software | less common |
Check them in order — the list is ranked so the likely, cheaper causes come before the expensive part.
Symptoms you may notice
- Check engine or electronic throttle light
- Limp mode: much reduced power, raised idle
- Delayed or jerky throttle response
- Often after a cold start in frost
Misdiagnosis traps
- Replacing the part named by the code title without testing. The code reports a failed self-test; the cause list above is ranked so the likely, cheap causes are checked before the expensive part is condemned.
- Clearing the code and declaring victory when the light stays off for a day — the monitor that sets this code runs only under specific driving conditions.
(General diagnostic guidance for this code family — no code-specific trap is published for this code.)
How a good shop confirms it
- Confirm the code and record any companion codes — companions narrow the system fast.
- Inspect the affected components and their wiring/connectors for damage, corrosion and looseness.
- Test the top-ranked cause with live data or a multimeter before replacing anything.
(Standard diagnostic path for this family, labeled as general guidance.)
Repair options & costs
| Repair | Shop total (parts + labor) | DIY (parts only) | Difficulty |
|---|
No single repair is costed for this code — the fix follows the diagnosis. The Repair Cost Estimator prices 28 common repairs once you know which one this is.
OBDex repair data for this code rates the job difficulty medium with a typical labor time in the 0.5–2 hour range and DIY possible: yes.
Frequently asked questions
How much does it cost to fix P0638?
There is no single fix — the code names a system, and the cost follows whichever cause on the ranked list proves out. The Repair Cost Estimator prices the common repairs in that system; diagnosis (often $100–$180, frequently credited) comes first.
Can I drive with P0638?
See the can-you-drive verdict above — it is based on the system involved and the cascading-damage risk, and it is our editorial triage, not a manufacturer instruction. If the light is flashing or the car feels unsafe, stop driving it regardless of any code.
Will P0638 clear itself?
If the fault was a one-off (a loose cap, a cold-start glitch), the light can go out after several clean trips and the code ages into history. A fault that is still present re-sets the code as soon as the car runs that self-test again — monitors run under specific conditions, which is why "it hasn't come back in two days" is not proof of a fix.
Will my car fail inspection with P0638?
In states with OBD-based emissions inspection, yes — an illuminated check-engine light or an incomplete readiness monitor fails the test regardless of what the tailpipe would show. C-codes (ABS) fall under safety inspection instead in states that have one.
What should I check first for P0638?
The first item on the ranked-causes list above — it is ranked likelihood-first for exactly this reason — plus the free checks: recent work in the area (connectors left loose are a classic), fluid levels, and the battery and grounds on electrical codes.
Where this data comes from
Code facts, causes and symptoms come from OBDex — an open dataset of independently authored descriptions of the standardized generic OBD-II codes (CC0 1.0, public domain; github.com/foerbsnavi/OBDex). The SAE J2012 document itself is copyrighted and is not reproduced here. Cost ranges are Number Nest's own US planning model: national parts and labor ranges cross-checked against the CarMD Vehicle Health Index 2026 published averages, with labor adjustable by the BLS OEWS May 2025 state wage index for auto technicians (occupation 49-3023) — a wage index, not a surveyed shop rate. Severity and can-you-drive notes are our editorial triage, not a manufacturer instruction.